feat: complete translator stack

backend/app/models/__init__.py

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+from . import session

backend/app/models/session.py

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+from datetime import datetime
+from typing import List, Optional
+
+from pydantic import BaseModel, Field
+
+
+class SessionState(BaseModel):
+    session_id: str
+    file_title: str
+    text_title: Optional[str]
+    source_lang: str
+    target_lang: str
+    total_chunks: int
+    current_chunk_index: int
+    status: str
+    started_at: Optional[datetime] = None
+    ended_at: Optional[datetime] = None
+    last_chunk_at: Optional[datetime] = None
+    eta_seconds: Optional[int] = None
+    chunks_remaining: Optional[int] = None
+    stop_requested: bool = False
+    error_message: Optional[str] = None
+    translated_text: str = ""
+    translated_chunks: List[str] = Field(default_factory=list)
+    final_document_path: Optional[str] = None